1. Field of the Invention
The present invention relates to a power transfer device in combination with a power transmission for automotive vehicles, and more particularly to a power transfer device of the type which includes a limited-slip differential arranged to effect limited-slip torque proportional to the difference in rotational speed between the front and rear wheel axles.
2. Discussion of the Background
In Japanese Patent Early Publication No. 60-236839 issued on Nov. 25, 1985, there has been proposed a power transfer device which includes a center differential of the bevel gear type arranged to split the driving power from a power transmission into two torque delivery paths respectively for front-wheel drive and rear-wheel drive, a front-drive differential of the bevel gear type arranged coaxially with the center differential at one side thereof to transfer the split driving power to front or rear-wheel axles, and an output ring gear for rear-wheel drive arranged coaxially with the center differential at the other side thereof to transfer the split driving power to rear or front-wheel axles. In the power transfer device, a limited-slip differential is assembled within a mounting case for the output ring gear to effect limited slip torque proportional to the difference in rotational speed between the front and rear-wheel axles. The limited-slip torque causes an increase of the driving torque at a low speed side and a decrease of the driving torque at a high speed side. This is effective to enhance roadability of the vehicle.
In the limited-slip differential, however, a rotary element is drivingly connected to the casing of the center differential, and another rotary element is drivingly connected to one of the side gears of the center differential. Namely, the limited-slip differential is interposed between the input element of the center differential and one of the output elements of the same. In such arrangement of the limited-slip differential, the limited-slip torque is distributed from the input element of the center differential to the other output element of the same. As a result, only one half of the limited-slip torque is effective to drive the wheel axles. It is, therefore, required to assemble a large size limited-slip differential for increasing the limited-slip torque two times. This causes disadvantages in a mounting space for the limited-slip differential and in weight and cost of the same.
In Automotive Engineering, Volume 93, Number 5, Page 91, there has been proposed a power transfer device which includes a planetary gear unit adapted as the center differential in the former power transfer device described above. The planetary gear unit includes an input element in the form of a carrier and output elements respectively in the form of a sun gear and a ring gear. In the power transfer device, a limited-slip differential is interposed between the output elements. However, the arrangement of the planetary gear unit is quite different from that of the center differential in the former power transfer device. For this reason, it is impossible to adapt the arrangement of the limited-slip differential in the latter power transfer device to the former power transfer device.